The works published by the Swiss psychologist Jean Piaget and his associates during the past forty years constitute the largest repository of knowledge about the cognitive development of children that is available anywhere, and Piaget's general theory of intellectual development rivals, in scope and comprehensiveness, Freud's theory of personality development Here is a self-contained general summary of Piaget's theory, written at a relatively nontechnical level. It is suitable for use in a variety of courses in psychology and education -- child psychology, child development, educational psychology, learning, psychological systems, general psychology, and others. It will also interest professionals and educated laymen as a timely exposition of ideas that are attracting the attention of increasing numbers of American psychologists. In order to convey the complexities of the theory to readers who have had no previous contact with it, the author uses a number of unusual pedagogical devices. He first outlines the theory in an introduction that students can reread with increasing comprehension as they study the text. The main part of the book is an elucidation of the Piagetian periods of intellectual development, with enough illustrations of Piaget's research activities to give the theory meaning. The author frequently reproduces passages from Piaget's clinical observations with Piaget's interpretations deleted, so that the reader can assess his own understanding and better appreciate Piaget's style of inquiry. In an epilogue, the author discusses the educational implications of Piaget's work.